The invention relates to an EUV lithography apparatus, and to a method for determining the contamination status of at least one EUV-reflective optical surface for at least one contaminating substance, the EUV-reflective optical surface being arranged in an EUV lithography apparatus.
Optical elements used in EUV lithography apparatuses are typically reflective elements, as no optical materials are known which can provide sufficient transmission for radiation at EUV wavelengths, typically in a range from about 5 nm to about 20 nm. In such EUV lithography apparatuses, it is necessary to operate the EUV-reflective optical elements in a vacuum because the service life of the EUV-reflective elements is limited by contaminating substances which may deposit on their EUV-reflective surfaces. In the context of the present application, contaminating substances are defined as substances which are susceptible to form deposits on the optical surfaces, and in particular when exposed to EUV-radiation. In this respect, hydrocarbons having an atomic mass of about 40 amu or above are considered as contaminating substances, whereas hydrocarbons having an atomic mass below 40 amu, e.g., methane (CH4), usually stay volatile even when irradiated with EUV light and do not form such deposits.
The contamination of EUV-reflective optical surfaces has an adverse effect on the performance of the EUV lithography apparatus as a whole, as such, contamination affects the optical properties of the optical components and may cause a loss in reflectivity or introduce wavefront errors. Therefore, control and knowledge of the contamination status of EUV-reflective optical elements is desired. In this respect, US 2004/0227102 A1, U.S. Pat. Nos. 6,847,463 B2, and 7,084,982 B2 disclose a measuring unit for determining the contamination status of an EUV-reflective optical surface, the measuring unit comprising a light source (e.g. a LED) for emitting light to the EUV-reflective optical surface, and an optical sensor for detecting the intensity of the light reflected from the EUV-reflective optical surface. In such a way, the contamination status, and in particular, the thickness of a contamination layer on EUV-reflective optical surfaces, may be determined.
For determining the reflectivity or transmission of optical elements for the visible wavelength range, it is known to use a technique which is referred to as cavity ringdown spectrometry. A cavity ringdown spectrometer typically comprises a laser source, an optical resonator, and a detection unit for detecting the laser radiation reflected or transmitted by the optical element, as described, e.g., on the website of the company “Los Gatos Research” www.lgrinc.com. Cavity ringdown spectroscopy involves measuring the decay of laser light in the optical resonator after the laser is rapidly switched off. The decay curve is an exponential curve with a time constant T from which the reflectivity or transmission of the optical element may be deduced. In such a way, the reflectivity and/or transmission of optical elements for the visible wavelength range can be determined with high precision.
As indicated in WO 2008/034582 A2, it is known to use cavity ringdown spectroscopy for the determination of partial pressures of contaminating gaseous species in an EUV lithography apparatus. In such case, the absorption of the gaseous species for the radiation inside the optical resonator is determined, thus allowing an artisan to deduce the concentration of the absorbing gaseous species in the resonator, and to gain information about the partial pressure of that gaseous species in the EUV lithography apparatus, and in particular, close to the EUV-reflective optical surfaces. However, for measuring the contamination status of the EUV-reflective optical surfaces themselves, WO 2008/034582 A2 suggests the approach described, e.g., in US 2004/0227102 A1, U.S. Pat. Nos. 6,847,463 B2, and 7,084,982 B2, cited above.